Cancer is one of the world nations' leading causes of death. In Japan, 300,000 or more people die of cancer every year, and its early detection and treatment are desired. Human deaths of cancer are mostly due to metastasis and recurrence of cancer. Metastasis and recurrence of cancer occur when cancer cells, which have passed through blood vessels or lymph vessels from a primary lesion, settle on and infiltrate the blood vessel wall of another organ tissue to form a minute metastasis lesion. Such cancer cells that circulate in a human body through blood vessels and lymph vessels are called circulating tumor cells (also referred to as “CTCs”, hereinbelow.)
In blood, blood cell constituents such as red blood cells, white blood cells, and platelets are much included, and the number thereof is said to be 3.5 to 9×109 per 1 mL of blood. There exist only few CTCs among these constituents. To efficiently detect CTCs from blood cell constituents, it was necessary to separate blood cell constituents, and observation and measurement were very difficult.
Cancer cells such as CTCs are one size larger than blood cells in blood, such as red blood cells, white blood cells, or platelets. Thus, it is theoretically possible to remove these blood cell constituents using a mechanical filtering method to thereby concentrate cancer cells. Since there exist cells having a size comparable to that of CTCs among white blood cells, there is a case where it is not possible to distinguish only CTCs at high precision depending only the size difference. However, since the deformability of white blood cells is higher compared to cancer cells, white blood cells can pass through holes smaller than themselves with external forces such as suction and pressurization, and thus it becomes possible to separate white blood cells from CTCs. It is contemplated to use a metal filter as a filter to perform the mechanical filtering method.
As a method for producing a metal filter, a electroforming plating method using photolithography is known.
For example, in Patent Literature 1, a method for producing a metal mask is described, wherein, after a first photosensitive resin layer is formed on a substrate having electrical conductivity, a first photomask on which a mesh pattern is formed is overlaid on the above-described first photosensitive resin layer and exposed, development treatment is performed to remove an unnecessary portion, a first plating layer is formed on the removed portion by electroforming using the above-described substrate as one electrode such that the thickness does not exceed the above-described first photosensitive resin layer, an electrically-conductive thin film is formed on the surface of the first plating layer and the above-described first photosensitive resin layer by a sputtering method, a second photosensitive resin layer is formed on the surface of the thin film, a second photomask on which a print pattern is formed is overlaid on the above-described second photosensitive resin layer and exposed, development treatment is performed to remove an unnecessary portion, and a second plating layer is formed on the removed portion by electroforming using the above-described thin film as one electrode such that the thickness does not exceed the above-described second photosensitive resin layer, the metal mask is formed by stripping the above-described substrate and removing the exposed portion of the first photosensitive resin layer, the second photosensitive resin layer, and the thin film.
Additionally, in Patent Literature 2, a method for producing a metal mask including a step of strippably laminating or applying a photoresist on a surface of a base composed of a flat plate, a step of overlaying a pattern film on the photoresist and exposing the photoresist to a light directing straight in perpendicular to the base, a step of stripping the pattern film and transferring the photoresist to a electroforming matrix side, a step of forming a pattern resist film on the electroforming matrix by performing development and dry treatment, a step of forming electrodeposited metal on a surface not covered with the pattern resist film of the electroforming matrix, and a step of stripping the electrodeposited metal from the electroforming matrix is described.